We studied organic thin film transistors based on poly-(3-hexylthiophene) having as gate dielectric Al 2 O 3, which was prepared by atomic layer deposition (ALD) technique, that provides films with very good electrical properties, roughness below 1 nm and compatibility with virtually any type of substrate, including polymeric ones. High-k gate oxides such as Al 2 O 3 are advantageous since they enable a reduction of operating voltages, but when used in conjunction with organic semiconductors, they induce worse transport properties if compared to low-k dielectrics. To address this issue, we focused on the interface between the gate dielectric and the active material and we explored the effects of functionalizing the Al 2 O 3 surface by means of self-assembled monolayers (SAM). We studied and compared a set of SAMs differing in the ligand groups and in the chain lengths. We show that the most important …